US-A 2008/0287807 A1 discloses a method for breast cancer screening. For the purpose of tumor detection, digital imaging of an actuated breast is employed to determine tissue surface motion. On the basis of said tissue surface motion, an internal stiffness distribution is reconstructed, whereby regions of high stiffness suggest cancer. The method according to US-A 2008/0287807 A1 comprises a step of placing a plurality of fiducial element markers on a tissue surface, a step of actuating the tissue surface, a step of imaging the tissue surface with an array of digital cameras, a step of choosing motion invariant properties of the fiducial element markers, a step of tracking the fiducial element markers from image to image and a step of using the tracked motion in each camera and the camera calibration to measure tissue surface motion.
During imaging procedures performed by e.g. Magnetic Resonance Imaging (MRI) or X ray devices, it is essential for the medical professional to be provided with accurate information regarding global movements of the patient's exterior. The method disclosed in US-A 2008/0287807 A1 is typically arranged for detecting local movements of the patient's exterior, but is not capable of detecting global movement of the patient's exterior.